How to Cope with Stress and Tension
Publication: Construction News 16 November 2006
Journalist: Alasdair Reisner
Kingston University called upon Dean and Dyball to build a six-storey building featuring lecture theatres, teaching rooms and IT suites. The project manager on the job, Nick Thurtle, tells Alasdair Reisner why a tight site presented no problems for the construction team.
YOUR FIRST challenge when visiting Dean and Dyball’s Quad site at Kingston University is finding it. Having walked around the entire perimeter of the campus you will notice a small road that slips almost unnoticed between the academic buildings. Surely this cannot be the only way on and off site for a £15 million construction project to deliver a six-storey building?
This is indeed the case. While the site entrance is tight, it provides access to a major site slap bang in the middle of the university estate. The project is not so much surrounded as bear-hugged by the working university with Dean and Dyball working cheek by jowl with the university’s various facilities.
You’d think this would create a nightmare for both contractor and university as each gets under the feet of the other, disrupting work and study in equal measure. But Dean and Dyball project manager Nick Thurtle says client and contractor are enjoying a peaceful co-existence.
“We had lots of meetings with the university when we began and developed a traffic management plan as a result of that,” says Mr Thurtle. “The original concept was for students to be able to walk across part of the site but we managed to move some classrooms to give us an efficient compound. We discuss what we will be doing on a month by month basis and what noisy activities we might be doing that might affect them. It also works the other way round for when they need quite periods for examination – it is just about understanding of each other’s needs and co-operation.”
The restricted nature of the site meant that it wasn’t difficult to decide what material would be used for the building.
“If you used steel you would have to have big articulated lorries coming in and there is just no space. No matter how many concrete wagons we have we can cope with the size of them. We got better flexibility with the delivery of concrete compared to steel,” says Mr Thurtle.
This was far from the only reason for choosing concrete. Perhaps the main one could be best illustrated by the adage that you can’t fit a quart into a pint pot. In other words, to get the full six-storey height with wide floor spans within the height allowed by planning the only realistic option was to go for a post-tensioned concrete solution.
James Molloy, contracts manager for concrete subcontractor Atlantic Contracts, explains: “post-tensioning reduces the amount of reinforcement in the building. It hopefully gives you a thinner slab and a longer span. If this was a residential there perhaps wouldn’t be the requirement for such long spans. You could have walls in between that allow them to be much shorter. But because of the large open spaces you need for the lecture theatres you couldn’t do it successfully without a post-tensioned solution.”
Post-tensioning was part of the original design provided by client’s agent Arup but as a design and build deal it was upon the contractor to bring any improvements it could to make the job more efficient.
“We were able to pass on some value engineering savings to the client as part of the process of winning the job. We managed a significant reduction in the number of sheer walls required for the job. But this also managed to reduce the loads being transferred down the structure so, as a consequence, we have been able to reduce the substructure and as a result the size of the piling,” says Mr Thurtle, highlighting the work Peter Brett Associates did to drive this change.
The team has been able to reduce the diameter of the piles from 900mm to 550mm, with a dramatic effect on the resources required.
“Once you start to reduce the pile diameter the required size of the pile caps reduces dramatically. There was a significant saving there,” says Mr Thurtle. Overall the pile caps were reduced from around 1.8m in depth to just 1.2m.
While these changes were worked out by the whole project team including Arup, Peter Brett Associates and Freyssinet, as Atlantic Contracts’ first job for Dean and Dyball the concrete subcontractor had a particular reason for trying to prove itself in the hope of winning further work in the future.
“We wanted to have a competitive edge so we went out to a couple of engineering firms and asked for their input at the early tender stage before Dean and Dyball had even secured the project themselves,” says Mr Molloy.
“We aimed to be in the position that if we were successful in winning the job we had something to offer Dean and Dyball that hopefully no one else had seen.”
Both sides say that the relationships between the two firms were further improved by the fact that the people involved in bidding for the work were the same as those who ended up running the job once it got the go-ahead.
“You do get preconstruction teams that don’t transfer onto the construction side. There is a handover and you’re left with someone else’s thoughts, whereas we were fortunate to take things right from the early design stage,” says Mr Molloy.
Mr Thurtle agrees: “We all had an interest in making it work. There was no blaming anyone else because we were the ones who put it together.”
Work on the scheme is due for completion in July 2007, at which point Dean and Dyball hopes it will be able to hand over the Quad to a university that has barely noticed the construction work required to build it.
Holding up the lecture
ON TWO of the six floors of the Quad building the university asked that Dean and Dyball create large open areas to provide space for lecture theatres. But this created an issue for the contractor. The design featured columns to support the floor above that would sit right in the middle of the theatres. The only way around this was to support from above rather than below, according to Mr Molloy.
“The way we got round it was to introduce a large beam from the level above so the floor beneath can be hung from it, clearing the storey below of columns,” he says.
The beams descend through the floor and have a flat plate attached to them which sits on the ceiling and provides the required support.
Post-tensioning at Kingston
THE PROCESS of forming the post-tensioned floors of the Quad building begins with the formation of the vertical wall and column elements.
Once these are poured and struck Atlantic was responsible for placing the falsework for the floor above, which will take the soffit board. With this done the team can then place the bottom mesh of reinforcement.
While the perimeter steel is being situated the Atlantic team can also be placing the tendon ducts for the post-tensioning ends.
With all this in place and edge shutters located the concrete can be poured. Twelve hours after the pour the tendons will receive their first stress, presuming the concrete has achieved 25 per cent of its strength, established by cube testing. After three days there is a second stress which gets the tension up to 100 per cent. Grout is then pumped into the tendon duct to fill any gaps so it can act as a single unit. The falsework is struck and work on the next floor can commence.